Los Angeles radio station KROQ is known for its early championing of alternative bands. As such, the station's annual Almost Acoustic Christmas concert, a two-night showcase held every December, is a pretty good litmus test for the state of rock music of the past year.

And what this year's fest – held this past weekend at L.A.'s Gibson Amphitheater – showed is just how diverse the alternative scene is now. It's one thing to hear the folk sounds of Mumford & Sons back-to-back with the engaging pop of Foster The People and the Naked and Famous, and to then hear the classic hard psychedelic rock of Jane's Addiction on the radio. It is quite another to see 6,000 kids roaring at an acoustic guitar solo, dancing to a techno-flavored remix and feeling rapturous pleasure at a band with a harp, but that was the case at night two for Mumford, Foster and Florence and The Machine, respectively.

Night one highlights included a performance from veterans Social Distortion, who played the station's first Acoustic Xmas 22 years ago. Bush led the nostalgic night, which also featured Blink-182 and 311. Sublime With Rome played their last show with drummer Bud Gaugh, who was leaving the band to focus on fatherhood. Yes, the Nineties were back, but with the caveat that every band had released a new album this year. Still, as expected, songs like Bush's "Everything Zen," Sublime's "Santeria" and 311's "Down" earned the night's biggest applause.

Night two also saw bits of nostalgia, from headliners Jane's Addiction and Noel Gallagher's High Flying Birds, who led the crowd in sing-alongs of Oasis staples "Wonderwall" and a gorgeous "Don't Look Back In Anger." But both acts also infused their sets with new material, firmly establishing that they are indeed relevant in 2011. To keep up with the rest of the participants in this night, they had to be, as this show also featured relative newcomers Black Keys and Florence and the Machine.

After highly engaging sets by L.A's own Grouplove and The Naked and Famous, Foster The People celebrated what frontman Mark Foster called "one of the best years of our lives," with a smashing hometown set punctuated by the current hit "Don't Stop (Color On The Walls)" and 2011's ubiquitous "Pumped Up Kicks."

Florence and the Machine, however, owned the evening. Speaking to Rolling Stone backstage, Bush's Gavin Rossdale heaped praise on Welch. "I was watching Florence, who I think is a complete goddess and just the most special musician that I’ve come across in a very, very long time," he said. "And when she sings, it is like she illuminates the world we all want to live in."

Welch showed exactly what Rossdale meant with songs such as "Dog Days Are Over," "Shake It Out," "Never Let Me Go," and the closing "No Light, No Light." Uniting and uplifting the crowd together as one, she showed she is ready to take the next step to festival headliner.

Mumford & Sons are already there, and they demonstrated why with a 40-minute set that not only featured hits like "Little Lion Man" and "The Cave," but two new songs, "Lover Of The Light" and "Ghosts That We Knew." The two new tunes showcased a sonically fuller sound, be it the soulful horns of "Lover Of The Light" or the more atmospheric "Ghosts."

Mumford is in the same position the Black Keys were in 2010, when they showed a lot of potential with some new material. When the Keys returned to this year's Acoustic Xmas with a new album, they cemented their place in the alternative rock pantheon.

And if 10 or 20 years from now night one of the two-night Acoustic Xmas is a showcase for the nostalgia of 2011, no one will likely complain or be surprised.